Life History Variation Among Four Lake Trout Morphs at Isle Royale, Lake Superior

Journal of Great Lakes Research 42 (2016) 421–432

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Journal of Great Lakes Research

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Life history variation among four lake trout morphs at Isle Royale, Lake Superior

Michael J. Hansen a,⁎, Nancy A. Nate b, Andrew M. Muir c, Charles R. Bronte d, Mara S. Zimmerman e, Charles C. Krueger b a U.S. Geological Survey, Great Lakes Science Center, Hammond Bay Biological Station, 11188 Ray Road, Millersburg, MI 49759, USA b Michigan State University, Center for Systems Integration and Sustainability, 1405 South Harrison Road, 115 Manly Miles Building, East Lansing, MI 48823, USA c Great Lakes Fishery Commission, 2100 Commonwealth Boulevard, Suite 100, Ann Arbor, MI 48105, USA d U.S. Fish and Wildlife Service, Green Bay Fish and Wildlife Conservation Ofﬁce, 2661 Scott Tower Drive, New Franken, WI 54229, USA e Washington Department of Fish and Wildlife, 1111 Washington Street Southeast, Olympia, WA 98501, USA article info abstract

Article history: Life history traits were compared among four morphs of lake trout at Isle Royale, Lake Superior. Of 738 lake trout Received 18 September 2015 caught at Isle Royale, 701 were assigned to a morph (119 humpers, 160 leans, 85 redfins, and 337 siscowets) Accepted 16 December 2015 using a combination of statistical analysis of head and body shape and visual assignment. On average, redfins Available online 22 January 2016 were longer (544 mm), heavier (1481 g), heavier at length, more buoyant, and older (22 years) than siscowets (519 mm; 1221 g; 19 years), leans (479 mm; 854 g; 13 years), and humpers (443 mm; 697 g; 17 years). On av- Communicated by Stephen Charles Riley erage, leans grew from a younger age at length = 0 and shorter length at age = 0, at a faster early growth rate to a fi Index words: longer asymptotic length than the other three morphs, while red ns grew at a slower instantaneous rate and Char humpers grew to a shorter asymptotic length than other morphs. On average, leans were longer (562 mm) Life history and older (15 years) at 50% maturity than redfins (427 mm, 12 years), siscowets (401 mm, 11 years), or humpers Survival (394 mm, 13 years). Life history parameters did not differ between males and females within each morph. We Maturity conclude that differences in life history attributes of lean, humper, redfin, and siscowet morphs of lake trout Demographics are consistent with differential habitat use in waters around Isle Royale, Lake Superior. Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.

Introduction offshore waters (e.g., Lake Michigan, Bronte et al., 2008;LakeErie, Markham et al., 2008). Many visually-distinct and easily-identified sympatric morphs of the Although lake trout morphological diversity in Lake Superior is cur- lake trout Salvelinus namaycush were recognized more than a century rently reduced from historical conditions, lean, siscowet (fat), humper, ago in the Laurentian Great Lakes of North America (Agassiz, 1850; and redfin morphs are currently recognized (Bronte et al., 2003; Goodier, 1981; Roosevelt, 1865). However, much of this morphological Burnham-Curtis and Smith, 1994; Khan and Qadri, 1970; Krueger and diversity was lost in the middle of the 20th century when over-fishing, Ihssen, 1995; Moore and Bronte, 2001; Muir et al., 2014, 2015; Peck, predation by the invasive sea lamprey Petromyzon marinus, and habitat 1975; Thurston, 1962). The lean morph uses shallow waters b50 m, is degradation caused lake trout stocks to collapse throughout much of the streamlined in form, relatively low in fat content, and mostly piscivo- basin (Hansen, 1999; Krueger and Ebener, 2004; Muir et al., 2012a). rous when N460 mm in length (Conner et al., 1993; Dryer et al., 1965; Subsequently, fishery managers attempted to restore lake trout stocks Ray et al., 2007).Thesiscowet(fat)morphisdeepbodiedwitharound- by stocking hatchery-reared fish, restricting fisheries, and controlling ed snout, uses deep waters N80 m, is high in fat content, and feeds in pe- sea lamprey populations. However, hatchery production and stocking lagic habitat by following diel vertical movements of Mysis diluviana and were focused on the lean form of lake trout that occupied shallow in- Coregonus species (Hrabik et al., 2006) and in benthic habitat on scul- shore waters where fisheries were focused. Recently, lake trout recov- pins (Cottus sp. and Myoxocephalus thompsonii) and burbot Lota lota ery plans have expanded to include other forms that occupy deep (Conner et al., 1993; Ray et al., 2007). The humper morph has a small head, short snout, short maxilla, large eye, short and narrow caudal peduncle, and often uses isolated offshore reefs surrounded by water ⁎ Corresponding author. Tel.: + 1 989 734 4768; fax: + 1 989 734 4494. N90 m in depth or steep-sided sloping banks. The redfin morph is the E-mail addresses: [email protected] (M.J. Hansen), [email protected] most full-bodied, with the largest head, snout, and eyes, longest and (N.A. Nate), [email protected] (A.M. Muir), [email protected] (C.R. Bronte), fi [email protected] (M.S. Zimmerman), [email protected] deepest caudal peduncle, and longer pelvic and pectoral ns than (C.C. Krueger). other morphs (Muir et al., 2014). Lean, fat, and humper morphs are

http://dx.doi.org/10.1016/j.jglr.2015.12.011 0380-1330/Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. 422 M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432 genetically distinct (Dehring et al., 1981; Goetz et al., 2010; Guinand et al., 2014; Zimmerman et al., 2006, 2007, 2009), lake trout were et al., 2003, 2012; Krueger et al., 1989; Page et al., 2004), with heritable sampled in the Isle Royale region of Lake Superior because of its remote physiological differences in fat content (Eschmeyer and Phillips, 1965; offshore location, national park status, and historical and recent ac- Goetz et al., 2010; Thurston, 1962), swim bladder gas retention counts of the occurrence of multiple morphs. Historically, the nearshore (Ihssen and Tait, 1974), and developmental rates of fertilized eggs and fishery for lean lake trout was significant in this region of Lake Superior, fry (Horns, 1985). Heritability of traits like fat metabolism (Goetz with less intensive fishing on deep-water morphs (Bronte and Sitar, et al., 2010) and swim-bladder gas that affect the ability to regulate 2008), although these fisheries caused abundance to decline (Wilberg depth through buoyancy compensation (Henderson and Anderson, et al., 2003; Bronte and Sitar, 2008). However, the rebound of lake 2002; Ihssen and Tait, 1974) indicates that some of the sympatric mor- trout abundance at Isle Royale in the 1970s suggested that this area phological diversity of lake trout in the Great Lakes has genetic origins was less perturbed than mainland areas. Remoteness of the island and does not reflect environmental plasticity (Hindar and Jonsson, and recent relatively light fishery exploitation led us to expect that 1993). lake trout populations in waters surrounding Isle Royale would Similar morphological diversity of lake trout also occurs in other more closely resemble historical populations than those along the North American lakes, including Great Bear Lake (Alfonso, 2004; mainland shoreline where hatchery fish were stocked and fisheries Blackie et al., 2003; Chavarie et al., 2013, 2014, 2015a), Great Slave were comparatively intense. Life history metrics from such lightly Lake (Zimmerman et al., 2006, 2009), and Lake Mistassini, Quebec exploited populations could provide useful benchmarks for restoration (Zimmerman et al., 2007). Morphological diversity in multiple lakes and future management of lake trout biodiversity elsewhere in the suggests that diversity is an important feature of the species that should Laurentian Great Lakes. be incorporated into recovery plans within the Laurentian Great Lakes (Hansen et al., 2012; Krueger and Ebener, 2004; Muir et al., 2015). Methods Further, the presence of lean, fat (i.e. siscowet-like), and humper-like morphs across North America suggests that common selection pres- Lake trout were sampled at Isle Royale (48°00′N, 88°50′W) during sures and ecological opportunities result in multiple morphs that are 15–20 August 2006 and 6–8 August 2007 (Fig. 1). Three depth strata more widespread than just in the Laurentian Great Lakes (Eshenroder, were sampled based on depths occupied by lean, humper, and siscowet 2008; Muir et al., 2015). Differences in life history (age, growth, and ma- lake trout morphs in Lake Superior (Moore and Bronte, 2001): 0–50- turity) among sympatric lake trout morphs from the Laurentian Great m = eight sets; 50–100-m = six sets; and 100–150-m = six sets Lakes (e.g., Rahrer, 1965; Burnham-Curtis and Bronte, 1996)andother (Muir et al., 2014). Gill nets were 183-m long by 1.8-m high, and North American lakes (Chavarie et al., 2015b,inreview;Hansen et al., made of multifilament nylon twine, with 30.5-m panels of stretch 2012, in press) may reflect unique use of ecological opportunities and mesh sizes ranging from 50.8 to 114.3 mm, in 12.7-mm increments. niche space by sympatric morphs that requires different management Based on girth–total length (TL) relationships for lake trout caught in strategies for each morph. similar gill nets in Lake Superior (Hansen et al., 1997), the range of In this study, multiple life history traits were compared among four mesh sizes used would enable wedging of lake trout ranging from morphs of lake trout at Isle Royale, Lake Superior, to determine if life small juveniles (222 mm TL) to large adults (827 mm TL). Nets were history attributes differed among morphs. As part of an ongoing study set on the lake bottom for ~24 h. Data collected from each fish included of lake trout diversity in North America (Hansen et al., 2012; Muir total length (TL = mm), weight in air (Wa = grams), weight in water

Fig. 1. Sampling locations for lake trout (solid triangles) at Isle Royale, Lake Superior, during 15–20 August 2006 and 6–8August2007. Reproduced from Muir et al. (2014). M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432 423

(Ww = grams) with the swim bladder deflated (for buoyancy), sex model (Campana, 1990). The biological intercept (sagittal otolith (male or female), and maturity status (immature or mature; Muir width = 0.137 mm; age-0 lake trout length = 21.7 mm; Hansen et al., 2014; Zimmerman et al., 2006). et al., 2012) was based on equations describing relationships between Digital full-body photographs of each specimen were captured in the length, age in days, and sagittal otolith width of age-0 lake trout field using methods described by Muir et al. (2012b). Specimens were (Bronte et al., 1995). assigned to a morph (Fig. 2) using a combination of statistical analysis Catch/net, length, weight, relative body condition, relative buoyan- of size-free head and body shape and visual assignment, as described cy, and age were each compared between males and females for each by Muir et al. (2014). When head and body model assignments differed morph separately, and then among morphs and depth strata for both and visual consensus was not achieved on an individual fish, that indi- sexes combined using general linear models (GLM), with main effects vidual was removed from further analyses (Muir et al., 2014). To use bi- for morph and depth strata and the interaction between morphs and ological information from fish that were not classified by Muir et al. depths (Zar, 1999). To normalize residuals of the GLM, catch/net, length, (2014), we assigned most remaining individuals to a morph by visual and weight were transformed into logarithms prior to analysis. Least- comparison to the library of statistical and visual assignments, as de- squares means (±SE) from the GLM were back-transformed from scribed by Hansen et al. (2012). logarithms into original units of measure for catch/net, length, and Sagittal otoliths were removed during field collections, placed in weight. To correct for size-related trends in body condition and buoyan- small tubes, and allowed to dry. Sagittal otoliths were used because cy, relative body condition was defined as residuals from the power re- they have been validated for age estimation of lake trout to an age of lationship between log10(Wa)andlog10(TL), and relative buoyancy was at least 50 years (Campana et al., 2008). One otolith from each fish defined as residuals from the power relationship between log10(Ww) was embedded in epoxy, and a thin transverse section (400 μm) was and log10(Wa). Mean catch/net, length, weight, relative body condition, cut, mounted on glass slides, polished, and imaged for age and growth relative buoyancy, and age were compared pairwise among morphs and assessment (Hansen et al., 2012). Annuli were counted by three inde- depths with Tukey's honestly significant difference test (Zar, 1999). To pendent readers using criteria described by Casselman and Gunn enable comparison of the body condition of Isle Royale lake trout to

(1992), and specimens were excluded from analysis if the coefficient other populations, the weight of a 500-mm fish (W500), the median of variation (CV) of age estimates was more than 5% (Campana, 2001). length of all fish sampled, was estimated from the weight-length rela- Age estimates were used to inform demarcation of growth increments, tionship for each morph. measured from the nucleus to the maximum ventral radius of the Growth in length with age was modeled using two parameteriza- otolith, and radial measurements at each annulus were used to back- tions of the Von Bertalanffy length-age model, which express growth calculate length-at-age using the biological intercept back-calculation in terms of five, rather than only three, life history parameters (Mooij et al., 1999; Quinn and Deriso, 1999): −KtðÞ−t0 Lt ¼ L∞ 1−e þ ε −ðÞω=L∞ t Lt ¼ L∞−ðÞL∞−L0 1−e þ ε:

These length-age models describe back-calculated length Lt (mm) at age t (years) as a function of age at length = 0 (t0 = years; incubation time of embryos from fertilization to hatching), length at age = 0

(L0 = mm; length at emergence from the egg), early annual growth rate (ω = L∞ × K = mm/year; Gallucci and Quinn, 1979), instantaneous

growth rate (K = 1/year) at which Lt approaches the theoretical maximum length (L∞ = mm), and residual error (ε). Parameters L∞, K, t0, L0,andω were estimated using nonlinear mixed-effect models (package ‘nlme’ in R; RCoreTeam,2014), with a fixed population effect (the average growth curve for the population from which individual fish were sampled), random individual effects (growth curves for individual fish sampled from the population), and sex or morph as a fixed factor (to compare average growth curves between sexes or among morphs; Vigliola and Meekan, 2009). Mixed-effects models are appropriate for modeling the within-group correlation of longitudinal, auto- correlated, and unbalanced data, such as back-calculated growth histories (Pinheiro and Bates, 2000). To compare growth between sexes and among morphs, log-likelihoods of full models (with sex or morph) were compared to reduced models (without sex or morph) in likelihood-ratio tests (Hosmer and Lemeshow, 2000). If a likelihood-ratio test was significant (P ≤ 0.05), growth parameters were compared among morphs using single-factor ANOVA and Tukey's honestly significant difference test (Zar, 1999). Length and age at maturity were estimated using logistic regression based on the maturity status of individual fish sampled at each length or age (immature = 0; mature = 1):

ðÞ¼ÀÁ1 : Fx −ðÞþ 1 þ e b0 b1x

fi Fig. 2. Lean, humper, redfin, and siscowet (fat) lake trout morphs from Lake Superior The model describes the likelihood F(x) that an individual sh was (images are scaled to the asymptotic length for each morph from Table 2). sexually mature at age or length x as a function of an intercept b0 and 424 M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432 the rate of increase in the probability of maturity with increasing age or Results length b1. Model parameters b0 and b1 and their standard errors were estimated using logistic regression on the logit transformation of the lo- Of 738 lake trout caught at Isle Royale, 701 were successfully gistic equation (Hosmer and Lemeshow, 2000). To compare maturity assigned to a morph, which included 119 humpers (63 females, 56 between sexes and among morphs, log-likelihoods of full models (with males), 160 leans (83 females, 76 males, 1 unknown sex), 85 redfins sex or morph) were compared to reduced models (without sex or (31 females, 54 males), and 337 siscowets (150 females, 185 males, 2 morph) in likelihood-ratio tests (Hosmer and Lemeshow, 2000). If the unknown sex). Otoliths were recovered from 685 fish for which a likelihood-ratio test was significant (P ≤ 0.05), maturity was assumed to morph was assigned, of which age and growth could be estimated for differ between sexes or among morphs. Length and age at 50% maturity 654 fish: 105 humpers, 151 leans, 79 redfins, and 319 siscowets. Oto-

(L50 and A50) were estimated for each morph as the absolute value of liths from the remaining 31 ﬁsh were too deformed for age and growth the ratio of the intercept to the slope, |b0/b1|. To test for differences in analysis because of vaterite formations (see Bowen et al., 1999)inone length and age at maturity between morphs, log-likelihoods were com- or more otolith planes, including 15% of humpers, 10% of leans, 7% of pared between full and reduced models for pairs of morphs. redﬁns, and 8% of siscowets. Males and females did not differ in mean length, weight, relative weight, buoyancy, age, growth, length-at-

15 0.02

0.01 12

0.00 9 -0.01

Catch/net 6 -0.02

3 Body Condition Relative -0.03

0 -0.04 Lean Humper Redfin Siscowet Lean Humper Redfin Siscowet Morph Morph 600 0.06

0.04 550

0.02 500 0.00 Total Length Total Length (mm) 450 Buoyancy Relative -0.02

400 -0.04 Lean Humper Redfin Siscowet Lean Humper Redfin Siscowet Morph Morph 1.8 22

1.6 20

1.4 18

1.2 16 Age (years) Age Weight (kg) Weight 1.0 14

0.8 12

0.6 10 Lean Humper Redfin Siscowet Lean Humper Redfin Siscowet Morph Morph

Fig. 3. Mean catch/net, total length, weight, relative body condition, relative buoyancy, and age (horizontal bars = mean; vertical bars = SE) of lean, humper, redfin, and siscowet lake trout morphs captured at Isle Royale, Lake Superior, during 15–20 August 2006 and 6–8 August 2007 (horizontal lines at the same position on the y-axis join means that do not differ, P N 0.05). M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432 425 maturity, or age-at-maturity for any of the four morphs (Electronic stratum, and less abundant than siscowets in the intermediate stratum, Supplementary Material (ESM) Table S1), so males were combined whereas redfins were less abundant than siscowets in the deep stratum with females for subsequent comparisons of metrics among morphs. (Fig. 4; Table ESM S3). Siscowets were significantly more abundant (higher catch/net) than Leans and humpers were significantly shorter and lighter, on aver- leans, humpers, or redfins at Isle Royale (Fig. 3; ESM Table S2). Leans did age, than redfins and siscowets at Isle Royale (Table 1; Fig. 3;ESM not vary significantly in abundance among depth strata, whereas Table S2). Mean length and weight of leans were greater in the shallow humpers were more abundant in intermediate and deep strata than in than the deep stratum, whereas mean length and weight of humpers, the shallow stratum, redfins were more abundant in the intermediate redfins, and siscowets did not vary significantly with depth (Fig. 4;ESM stratum than in the deep stratum, and siscowets were more abundant Table S3). Morphs did not differ significantly in mean length or weight in the intermediate stratum than in the shallow stratum (Fig. 4; ESM in the shallow stratum (except leans were lighter than siscowets), where- Table S3). Leans were more abundant than humpers in the shallow as leans and humpers were shorter and lighter than redfins and siscowets

40 0.04

0.02 30

0.00 20 -0.02 Catch/Net

10 -0.04 Relative Body Relative Body Condition

0 -0.06 <50 <50 <50 <50 <50 <50 <50 <50 >100 >100 >100 >100 >100 >100 >100 >100 50-100 50-100 50-100 50-100 50-100 50-100 50-100 50-100 Lean Humper Redfin Siscowet Lean Humper Redfin Siscowet 600 0.28

0.20 550

0.12 500 0.04 Total Length (mm) Length Total 450 Relative Buoyancy -0.04

400 -0.12 <50 <50 <50 <50 <50 <50 <50 <50 >100 >100 >100 >100 >100 >100 >100 >100 50-100 50-100 50-100 50-100 50-100 50-100 50-100 50-100 Lean Humper Redfin Siscowet Lean Humper Redfin Siscowet 1.9 25

1.7

1.5 20 1.3

1.1 Weight (g) Weight Age (years) Age 15 0.9

0.7

0.5 10 <50 <50 <50 <50 <50 <50 <50 <50 >100 >100 >100 >100 >100 >100 >100 >100 50-100 50-100 50-100 50-100 50-100 50-100 50-100 50-100 Lean Humper Redfin Siscowet Lean Humper Redfin Siscowet

Fig. 4. Mean catch/net, total length, weight, relative body condition, relative buoyancy, and age (horizontal bars = mean; vertical bars = SE) of humper, lean, redﬁn, and siscowet lake trout morphs captured in three depth strata at Isle Royale, Lake Superior, during 15–20 August 2006 and 6–8August2007. 426 M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432

Table 1

Total length (TL = mm), weight in air (Wa = g), weight in water (Ww = g), age (years), relative body condition (condition = residuals of linear regression of log10(Wa) against log10(TL)), relative buoyancy (buoyancy = residuals of regression of log10(Ww) against log10(Wa)), age-at-length-zero (t0 = years), length-at-age-zero (L0 = mm), early growth rate (ω = mm/year), instantaneous growth rate (K = 1/years), asymptotic length (L∞ = mm), length at 50% maturity (L50 = mm), and age at 50% maturity (A50 = years) for 119 humper, 160 lean, 85 redﬁn, and 337 siscowet lake trout morphs captured at Isle Royale, Lake Superior, during 15–20 August 2006 and 6–8 August 2007. Test statistics are F-ratios for TL, Wa, Ww,age,condition,and 2 buoyancy, and Chi-squares (χ ) for growth (t0, L0, ω, K,andL∞) and maturity (L50 and A50).

Trait Parameter Lean Humper Redﬁn Siscowet Test P

TL Estimate 469 454 539 523 21.07 0.000 SE 6.6 11.3 14.0 5.5 Range 253–780 295–687 303–868 285–781

Wa Estimate 785 741 1386 1257 30.28 0.000 SE 37.4 62.1 121.1 44.2 Range 125–4200 160–3250 200–5300 150–5100

Ww Estimate 63 55 57 54 1382.02 0.000 SE 1.3 1.9 2.0 0.9 Range 8.0–220 5.5–132 11.0–234 8.5–200 Age Estimate 12.6 17.2 19.9 18.9 29.68 0.000 SE 0.562 1.033 1.023 0.416 Range 4–43 4–46 5–48 6–47 Condition Estimate −0.033 −0.012 0.007 0.009 23.47 0.000 SE 0.004 0.007 0.008 0.003 Buoyancy Estimate 0.031 −0.019 −0.045 −0.017 8.40 0.000 SE 0.009 0.016 0.017 0.007

t0 Estimate −0.665 −0.934 −1.094 −0.987 1.85 0.604 SE 0.0565 0.0688 0.0781 0.0390

L0 Estimate 41.6 47.5 55.9 51.5 2.99 0.393 SE 2.45 2.90 3.30 1.65 ω Estimate 71.1 59.8 58.4 59.0 94.90 0.000 SE 1.17 1.37 1.55 0.77 K Estimate 0.105 0.110 0.089 0.095 70.93 0.000 SE 0.00253 0.00295 0.00323 0.00161 L∞ Estimate 715 556 675 639 132.59 0.000 SE 11.7 13.2 15.2 7.5

L50 Estimate 562 394 427 401 131.13 0.000 SE 25 53 24 28

A50 Estimate 15.0 12.9 11.9 11.5 7.346 0.062 SE 0.9 2.9 2.2 1.4

in intermediate and deep strata (except leans did not differ in mean younger than siscowets in the deep stratum (Fig. 4; Table ESM S3). length from siscowets in the intermediate stratum; Fig. 4; ESM Table S3). The range in ages and the maximum ages observed were similar Leans were signiﬁcantly lower in mean relative body condition than among all morphs (Table 2). redﬁns and siscowets, and humpers were lower than siscowets at Isle Morphs differed in growth at Isle Royale (Table 1; Figs. 5–6; ESM

Royale (Table 1; Fig. 3; ESM Table S2). Leans and redfins did not differ sig- Table S2). Leans grew from a younger age at length = 0 (t0) and shorter nificantly in mean body condition among depth strata, whereas humpers length at age = 0 (L0), at a faster early growth rate (ω)toalonger were higher in body condition in the intermediate stratum than in the asymptotic length (L∞) than the other three morphs, while redfins deep stratum, and siscowets were higher in body condition in shallow grew at a slower instantaneous growth rate (K) and humpers grew and intermediate strata than in the deep stratum (Fig. 4; ESM Table S3). to a shorter asymptotic length (L∞) than other morphs at Isle Royale.

Leans were significantly lower in mean relative body condition than Mean t0 was higher for leans than for humpers, redfins, and siscowets in all three depth strata, and leans were lighter than all three siscowets. Mean L0 waslowerforleansthanforredfins and other morphs in the intermediate stratum (Fig. 4;ESMTableS3). siscowets, but not for humpers. Instantaneous growth rate K was Leans were significantly less buoyant than redfins or siscowets at Isle Royale, whereas humpers did not differ in mean buoyancy from the other three morphs, and redfins and siscowets did not differ in mean Table 2 β buoyancy (Table 1; Fig. 3; ESM Table S2). Leans did not differ significant- Slope of log10-transformed weight-length relationships ( ), predicted weight of a 500- fi ω ly in mean buoyancy among depth strata, whereas the other three mm sh (W500 = kg), asymptotic length (L∞ = mm), early growth rate ( = mm/year), length at 50% maturity (L50 = mm), and age at 50% maturity (A50 = years) for native lake morphs were more buoyant in the deep stratum than in the intermedi- trout populations from North America (mean, standard deviation = SD, coefficient of var- ate stratum, and redfins and siscowets were more buoyant in the deep iation = CV, and number of populations = N; Dubois and Lagueux, 1968; Hansen et al., stratum than in the shallow stratum (Fig. 4; ESM Table S3). Morphs 2012; Healey, 1978; Martin and Olver, 1980; McDermid et al., 2010; Mills et al., 2002; did not differ significantly in mean buoyancy in either shallow or inter- Piccolo et al., 1993; Shuter et al., 1998;andTrippel, 1993) and percentiles of each metric's fi mediate strata, whereas redfins were more buoyant than the other cumulative North American distribution for humper, lean, red n, and siscowet lake trout morphs captured at Isle Royale, Lake Superior, during 15–20 August 2006 and 6–8August three morphs and siscowets were more buoyant than leans in the 2007 (this table; this study). deep stratum (Fig. 4; ESM Table S3). β ω Leans were significantly younger than the other three morphs at Isle Source Statistic/Morph W500 L∞ L50 A50 Royale, none of which differed significantly in mean age (Table 1; Fig. 3; North America Mean 3.228 1.136 639.6 107.2 421.9 8.9 ESM Table S2). Leans, humpers, and siscowets did not differ significantly SD 0.258 0.143 139.3 27.1 80.9 3.3 fi CV 8% 13% 22% 25% 19% 37% in mean age among depth strata, whereas red ns were older in the in- N 48 48 186 56 132 132 termediate stratum than in the shallow stratum (Fig. 4;ESMTableS3). Isle Royale Lean 48% 14% 71% 9% 96% 97% Leans were significantly younger than siscowets in all three depth stra- Humper 92% 34% 27% 4% 36% 88% ta, redfins were older than the other three morphs in the intermediate Redfin 78% 43% 60% 4% 52% 82% stratum, and humpers were younger than redfins and leans were Siscowet 56% 35% 50% 4% 40% 78% M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432 427

Fig. 5. Length versus age for humper, lean, redfin, and siscowet lake trout morphs captured at Isle Royale, Lake Superior, during 15–20 August 2006 and 6–8August2007(solidcircles= length at age of capture; dashed line = mean back-calculated length at age; dotted lines = 95% confidence limits for mean back-calculated length at age; solid lines = Von Bertalanffy length-age model estimated from individual back-calculated growth histories). faster for humpers and leans than for redfins and siscowets. Early asymptotic length than the sympatric lean morph. Likewise, the growth rate ω was faster for leans than for the other three morphs. siscowet-like morph in Great Slave Lake was older than the sympatric Asymptotic length L∞ ranged from a low of L∞ = 556 mm for humpers, lean morph (Hansen et al., in press). By contrast, length and age at ma- to L∞ = 639 mm for siscowets, L∞ = 675 mm for redfins, and a high of turity did not differ between fat and lean morphs in Great Slave Lake L∞ = 715 mm for leans. (Hansen et al., in press), whereas the fat morph matured at a shorter Morphs differed in length and age at maturity at Isle Royale (Table 1; length and older age than the lean morph in southern Lake Superior Fig. 7; Table ESM S1). Maturity was significantly related to length of (Sitar et al., 2014) and a shorter length and younger age than the lean leans (χ2 =24.5;df =1;P b 0.001), humpers (χ2 = 7.90; df =1; morph at Isle Royale (this study). In Lake Mistassini, Quebec, the P = 0.005), redfins (χ2 =31.4;df =1;P b 0.001), and siscowets humper morph was smaller bodied, older, and grew slower to a shorter (χ2 =27.2;df =1;P b 0.001). Leans were longer at 50% maturity asymptotic length the lean morph (Hansen et al., 2012), similar to dif- (562 mm) than redfins (427 mm), siscowets (401 mm), and humpers ferences between humper and lean morphs at Isle Royale, Lake Superior (394 mm years). Maturity was also significantly related to age of leans (Burnham-Curtis and Bronte, 1996; Rahrer, 1965; this study). Among (χ2 =36.6;df =1;P b 0.001), humpers (χ2 =10.8;df =1;P = the three morphs that occur elsewhere (lean, humper, and siscowet), 0.001), redfins (χ2 =25.4;df =1;P b 0.001), and siscowets (χ2 = the siscowet morph is the plumpest and oldest, but sometimes interme- 40.5; df =1;P b 0.001). Leans were older at 50% maturity (15 years) diate in length, whereas the humper morph is the smallest, but inter- than siscowets (11 years), but not humpers (13 years) or redfins mediate in body condition and age, and the lean morph is often the (12 years). longest, but leaner and younger than the other two morphs. This pat- tern appears to hold, even when a pair of morphs are relatively large Discussion bodied, such as siscowet-like and lean morphs in Great Slave Lake, Northwest Territories (Hansen et al., in press) or relatively small bodied, Differences in life history characteristics of humper, lean, and such as humper and lean morphs in Lake Mistassini, Quebec (Hansen siscowet morphs of lake trout at Isle Royale were generally consistent et al., 2012). with previous comparisons between pairs of these morphs elsewhere We assumed that sampling represented the full range of life history in Lake Superior and in North America. For example, a siscowet-like attributes of lake trout populations around Isle Royale, because (1) spa- (i.e., fat) morph in Great Slave Lake, Northwest Territories (Hansen tial coverage of sampling included a wide range of depths from multiple et al., in press) and in western Lake Superior (Miller and Schram, areas around the island and (2) sampling gear included a wide range of 2000) was also heavier, older, and grew more slowly toward a shorter mesh sizes to target the full range of possible lake trout sizes (Hansen 428 M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432

Fig. 6. Parameter estimates (horizontal bars = mean; vertical bars = 95% confidence limits) for two versions of the Von Bertalanffy length-age model estimated from individual back- calculated growth histories for humper, lean, redfin, and siscowet lake trout morphs captured at Isle Royale, Lake Superior, during 15–20 August 2006 and 6–8 August 2007 (horizontal lines at the same position on the y-axis join means that do not differ, P N 0.05). et al., 1997). Nonetheless, sampling in late summer could have missed from around Isle Royale (Coberly and Horrall, 1980; Goodier, 1981; pelagic and migratory components of populations that were present Loftus, 1980; Organ et al., 1979; Rakestraw, 1968; Toupal et al., 2002; in areas sampled at other times of the year or above nets. Such popula- Stuart Siverson, Duluth Minnesota, personal communication), which tions are currently unknown, and would require sampling throughout suggests that we may have encountered only the most abundant or the year to identify. Further, although areal and bathymetric coverage widespread morphs during our sampling. We therefore acknowledge of sampling was extensive, sampling locations were chosen opportunis- that our samples reflect only the main forms of lake trout present at tically, rather than based on a statistical sampling design. Therefore, in Isle Royale and that morphological diversity of lake trout may be greater an area as large and physically complex as Isle Royale, our sampling like- than we describe herein. ly missed small local populations of morphs that would have been en- Traits of the lean lake trout morph at Isle Royale were consistent countered by more spatially and temporally extensive sampling. For with a lightly exploited fish population. Body condition of lean lake example, numerous lake trout morphs have been described anecdotally trout at Isle Royale was low, because the predicted weight of a 500- M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432 429

Fig. 7. Proportion mature versus length (upper-left panel) and age (upper-right panel), and length (lower-left panel; horizontal bars = mean; vertical bars=95%confidence limits) and age (lower-right panel; horizontal bars = mean; vertical bars = 95% confidence limits) at 50% maturity for humper, lean, redfin, and siscowet lake trout morphs captured at Isle Royale, Lake Superior, during 15–20 August 2006 and 6–8 August 2007 (horizontal lines at the same position on the y-axis join means that do not differ, P N 0.05).E

mm ﬁsh (W500) was lower than 86% of 48 lake trout populations in 1965), but intermediate between humper-like morphs in Lake Mistassi- North America (Table 2). Lean lake trout at Isle Royale grew slower ni (W500 = 0.947 kg; Hansen et al., 2012) and Rush Lake (W500 = (ω) than 91% of 56 populations toward an asymptotic length (L∞)that 1.133 kg; Chavarie et al., in review). The humper morph at Isle Royale was longer than 71% of 186 populations in North America (Table 2), as in 2006–2007 (this study) was similar in mean age as in 1989–1992 expected for a high-density population (Rose et al., 2001). Length and (mean age = 16 years; range = 8–29 years; Burnham-Curtis and age at maturity of lean lake trout at Isle Royale were higher than 96% Bronte, 1996) and in Rush Lake (mean age = 16.8 years; range = of 132 populations in North America (Table 2), as expected for slow- 7–31 years; Chavarie et al., in review), but younger than in Lake Mistas- growing populations (Healey, 1978). In contrast, lean lake trout at Isle sini (mean age = 27 years; range = 13–49 years; Hansen et al., 2012). Royale were not as large or as old at maturity as four shallow-water The humper morph at Isle Royale grew at a slower early rate (ω)toa morphs in Great Bear Lake (L50 = 582–704 mm, A50 = similar asymptotic length (L∞) in 2006–2007 (this study) as in 17.4–20.2 years, Chavarie et al. 2015b), as expected for a slow- 1989–1992 (ω = 116 mm/year, L∞ = 542 mm, SE = 13; Burnham- growing population in an unproductive lake at a northerly latitude Curtis and Bronte, 1996), Lake Mistassini (ω = 52.3 mm/year, SE = with a short growing season (McDermid et al., 2010). Collectively, life 2.15; L∞ = 514 mm, SE = 14.1; Hansen et al., 2012), and Rush Lake history attributes of lean lake trout at Isle Royale are consistent with (ω = 53.0 mm/year, SE = 2.33; L∞ = 489 mm, SE = 20.0; Chavarie low exploitation (old aged) and high density relative to prey supply et al., in review). The humper morph at Isle Royale in our study matured

(low body condition), which has been declining (Gorman et al., 2013), at a similar size and age as in Lake Mistassini, Quebec (L50 = 395 mm; so growth is slow toward a large asymptotic size, and large size and A50 =13years;Hansen et al., 2012), has a smaller size and an older old age at maturity (Rose et al., 2001). age than in Rush Lake (L50 = 423 mm; A50 = 10 years; Chavarie et al., Traits of the humper lake trout morph at Isle Royale were generally in review), and has a larger size than at Isle Royale in 1959–1963 similar to the same morph elsewhere in North America. Size struc- (L50 =343–359 mm; Rahrer, 1965). Eshenroder (2008) theorized ture of the humper morph in our study was similar to an earlier that a humper-like morph diverged postglacially in sympatry from the study (1959–1963, mean = 442 mm; range = 376–742 mm; Rahrer, ancestral lean morph as a feeding specialist on Mysis. 1965) and in Lake Mistassini, Quebec (mean = 474 mm; range = The redfin morph recently described by Muir et al. (2014) was the 389–616 mm; Hansen et al., 2012), but larger than a humper morph largest, oldest, and slowest-growing morph at Isle Royale, Lake Superior, in a relatively small lake adjacent to Lake Superior (Rush Lake, Michi- similar to early qualitative descriptions of lake trout morphological gan, USA, mean = 393 mm; range = 335–641 mm; Chavarie et al., in diversity from the Laurentian Great Lakes. A redfin morph was often inreview). The humper morph was less plump in our study (W500 = cluded in early descriptions of lake trout morphological diversity from 1.078 kg) than in 1959–1963 at Isle Royale (W500 = 1.231 kg; Rahrer, Lake Superior (Rakestraw, 1968; Organ et al., 1979), and also from 430 M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432 lakes Michigan and Huron (Coberly and Horrall, 1980; Loftus, 1980). As 2003; Moore and Bronte, 2001; Bronte and Sitar, 2008), although the we found in this study, redfins in early accounts were described as being siscowet morph also occurs at much greater depth than the lean relatively large bodied in Lake Michigan (5–7 kg, Coberly and Horrall, morph (Sitar et al., 2008), largely because the siscowet morph metabo- 1980), Lake Huron (≥5kg,Loftus, 1980), and at Isle Royale in Lake Supe- lizes lipids differently, such that high concentrations are retained within rior (≤20 kg Toupal et al., 2002). The redfin morph described by Muir the flesh and organs (Goetz et al., 2010), which allow them to expend et al. (2014) was consistent in appearance with redfins described in less energy to remain buoyant at great depth (Henderson and early accounts: large bodied, with a big head and large pectoral fins, Anderson, 2002). High lipid content provides positive buoyancy that en- red flesh, red or red and yellow fins, sometimes spotted, and often ables the siscowet morph to migrate vertically in pursuit of invertebrate dark dorsal coloration (Loftus, 1980; Goodier, 1981). The life history of (M. diluviana)andfish (cisco and sculpin) prey (Ahrenstorff et al., 2011; redfins we quantified at Isle Royale may be maintained by reliance on Gamble et al., 2011; Isaac et al., 2012; Hrabik et al., 2014). Diets of a different (moderate) depth range than the other three morphs, as siscowet and lean morphs do not greatly overlap (Conner et al. 1993; was described earlier for redfins in Lake Huron (Loftus, 1980). Redfins Ray et al., 2007), because the siscowet morph exploits prey in both may also maintain distinct life history features by spawning at a differ- deep and shallow water (Harvey et al., 2003), thereby minimizing tro- ent time of year than other morphs, as can be inferred from commer- phic overlap between the two morphs in Lake Superior when measured cial fishery harvests of spawning redfins in late summer (August to using stable isotopes (Harvey and Kitchell, 2000). Similar studies of mid-September), similar to the spawning season for humpers humper and redfin morphs, in relation to lean and siscowet morphs, (Rahrer, 1965), but earlier than the late-autumn (October–November) are needed to confirm the roles of ecological and genetic differences in spawning period of leans (Coberly and Horrall, 1980; Goodier, 1981; maintaining morphological and life history diversity of lake trout at Loftus, 1980; Organ et al., 1979; Toupal et al., 2002). Similarly, a Isle Royale and across Lake Superior. redfin-like morph was observed by the authors in mature and spent Life history differences among lake trout morphs at Isle Royale sug- condition in August 2013 at Superior Shoal, a remote shoal 50 miles gest that lake trout restoration in the rest of the Laurentian Great Lakes east of Isle Royale. In Lake Huron, redfins were believed to spawn in should incorporate more than just a lean form. The remarkable diversity late summer over rocky substrate at depths of 1–18 m (Loftus, 1980), of lake trout forms at Isle Royale could serve as a reservoir for develop- and in Lake Superior, redfins often spawned on the same shoals used ing hatchery programs to support reintroduction of up to four morphs by leans in autumn (Goodier, 1981). Redfins spawn around Isle Royale elsewhere in the Laurentian Great Lakes (Krueger et al., 1995). For at Passage Island, Gull Island, and from Rainbow Point to Siskiwit Island example, rehabilitation plans for lake trout have recently aimed at over numerous shoals in 9–15 m of water, and were harvested at 13 reintroducing a humper morph into Lake Michigan (Bronte et al., areas around Isle Royale, including Redfin Island, named for the preva- 2008) and Lake Erie (Markham et al., 2008), and are now being stocked lence of the morph (Fig. 1; Toupal et al., 2002). in these lakes as well as in Lake Ontario. Lake trout restoration that has Traits of the siscowet lake trout morph at Isle Royale were generally relied on stocking only the lean morph will use less than 40% of total similar to fat morphs elsewhere in North America and Lake Superior. available Great Lakes habitat (Eshenroder and Burnham-Curtis, 1999). Size structure of the siscowet morph at Isle Royale in our study was sim- Further, high historical diversity of lake trout morphology in waters ilar to the siscowet morph in southern Lake Superior (mean = 545 mm; around Isle Royale (Rakestraw, 1968; Toupal et al., 2002) suggests range = 340–828 mm; Sitar et al., 2014) and a siscowet-like morph in that Isle Royale is a unique area in the Laurentian Great Lakes basin Great Slave Lake, Northwest Territories (mean = 583 mm; range = that deserves careful consideration for conservation of lake trout diver- 308–802 mm; Hansen et al., in press). Similarly, mean age of the sity. Lastly, regulation of fisheries must acknowledge that harvest of siscowet morph at Isle Royale in our study was similar to the siscowet mixed stocks (morphs), such as those around Isle Royale or in areas of morph in southern Lake Superior (mean age = 19.0 years; Sitar et al., the basin where multiple morphs are introduced, will require mixed- 2014), but younger than a siscowet-like morph in Great Slave Lake stock management to avoid over-exploitation of weak stocks (Ricker, (mean age = 24.9 mm; Hansen et al., in press). The siscowet morph 1958). Life history attributes of lightly exploited populations of lean, inourstudywaslessplump(W500 =1.080kg)thaninwestern humper, redfin, and siscowet lake trout morphs at Isle Royale can be Lake Superior (W500 =1.768kg;Miller and Schram, 2000), but sim- used as benchmarks for restoration and management plans of such ilar to a siscowet-like morph in Great Slave Lake (W500 =1.182kg; lake trout morphs elsewhere in the Laurentian Great Lakes. Hansen et al., in press). The siscowet morph at Isle Royale grew to a shorter asymptotic length (L∞) in our study than in western Lake Acknowledgments Superior (L∞ = 850 mm; Miller and Schram 2000), and at a slower early growth rate (ω) toward a longer asymptotic length than in We thank Stewert Sivertson and Enar and Betty Strom for their Great Slave Lake (ω = 54.1 mm/year, SE = 1.09; L∞ = 726 mm, knowledge of Isle Royale, Lake Superior, and its fishes, and for their hos- SE = 17.1; Hansen et al., in press). The siscowet morph at Isle Royale pitality at Washington and Barnam islands. Jonathan Pyatskowit and in our study matured at a similar size (L )andage(A )asinsouthern 50 50 Henry R. Quinlan (U.S. Fish and Wildlife Service), and Jay D. Glase Lake Superior (L = 443 mm, 95% CI = 408–462; A = 11 years, 95% 50 50 (U.S. National Park Service) provided cheerful and able assistance with CI = 8–13; Sitar et al., 2014), but a smaller size and younger age than in field work. Thanks to Scott Miehls (U.S. Geological Survey) and Sarah Great Slave Lake (L = 504 mm, 95% CI = 467–529; A = 16 years, 50 50 Seegert and Christina Haska (Great Lakes Fishery Commission) for their 95% CI = 14–17; Hansen et al., in press). assistance collecting morphological data. The Fishery Research Program Lake trout morphs differed in life history attributes at Isle Royale, de- of the Great Lakes Fishery Commission provided funding. Use of trade, spite relatively small morphometric (Moore and Bronte, 2001; Muir product, or firm names is for descriptive purposes and does not imply en- et al., 2014) and genetic (Baillie et al., in review)differencesamong dorsement by the U.S. Government. The findings and conclusions in this the same morphs. We therefore conclude that conservation of life histo- article are those of the authors and do not necessarily represent the ry attributes is ecologically important for conserving morphometric and views of the U.S. Fish and Wildlife Service. This article is Contribution genetic differences. For example, phenotypic differences we observed 2004 of the U.S. Geological Survey, Great Lakes Science Center. between lean and siscowet morphs at Isle Royale, such as body condition and buoyancy, persisted in lean and siscowet morphs from Lake Superior when reared under identical (common garden) conditions, Appendix A. Supplementary data which indicates that these differences are at least partially under genetic control (Eschmeyer and Phillips, 1965; Goetz et al., 2010). Siscowet and Supplementary data to this article can be found online at http://dx. lean morphs occupy overlapping depths in Lake Superior (Bronte et al., doi.org/10.1016/j.jglr.2015.12.011. M.J. Hansen et al. / Journal of Great Lakes Research 42 (2016) 421–432 431

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